Choosing Debian over Ubuntu avoids the additional bloat that Ubuntu has over Debian, and frees Valve from having to work with Canonical. I have a lot of respect for what Canonical's done to make Linux viable as a desktop OS, but Valve needs the freedom to unhitch its wagon from a for-profit company like Canonical when their goals don't coincide. No need for Unity in Steam OS, and Valve may see a future need to choose Wayland over Mir as the future display server.